President Trump has embarked on the most aggressive campaign against government regulation in a generation, joining with Republican lawmakers to roll back rules already on the books and limit the ability of federal regulators to impose new ones.

After just a few weeks in office, the new administration is targeting dozens of Obama-era policies, using both legislative and executive tactics. The fallout is already rippling across the federal ­bureaucracy and throughout the U.S. economy, affecting how dentists dispose of mercury fillings, how schools meet the needs of poor and disabled students, and whether companies reject mineral purchases that fuel one of the world’s bloodiest conflicts.

The campaign has alarmed ­labor unions, public safety advocates and environmental activists, who fear losing regulations that have been in place for years, along with relatively new federal mandates. Business groups, however, are thrilled, saying Trump is­ responding to long-standing complaints that a profusion of federal regulations unnecessarily increases costs and hampers their ability to create jobs.

Under Trump, “there’s great optimism that all of them will be addressed,” said Rosario Pal­mieri, vice president for labor, legal and regulatory policy at the National Association of Manufacturers.

Trump and congressional Republicans are working to strip rules away at an unprecedented rate. One of the most powerful levers is the Congressional Review Act, a 1996 law that gives lawmakers the power to nullify any regulation within 60 days of enactment.

Before Trump took office, the Congressional Review Act had been successfully used only once, to overturn a Clinton administration ergonomics rule in 2001. So far this year, the House has moved to nullify eight new rules and is considering dozens more. Two of those measures — which would loosen environmental restrictions on waste-mining companies and financial disclosure requirements on oil and gas firms — have cleared the Senate and are on their way to the White House for the president’s signature.

That sets a far more stringent standard than recent Republican administrations have attempted, experts on regulation said, leaving a slew of Obama-era rules in limbo.

“It’s clear as can be that they intend to reduce the level of regulation,” said James Gattuso, a senior fellow in regulatory policy at the conservative Heritage Foundation, who said the directive marks the first explicit attempt to contain the costs of federal mandates.

“If successful,” Gattuso said, “it would be the first time in a generation,” since Ronald Reagan was in the White House.

According to the White House Office of Management and Budget, the cost of federal regulations has grown every year since 1982. Republicans of all stripes have long railed against what they say are crippling economic effects.

Making sure government rules “are meeting their intent and not stifling job creation at the expense of whatever they were intended to do is something that should be smart and welcome by everybody,” he said.

“It’s a much more aggressive rollback attempt than we’ve seen in recent years,” said Tevi Troy, who served George W. Bush as a senior White House official and in two Cabinet-level agencies. He noted that many conservatives have long been disappointed that the Bush administration did not do more to “clear out some of the regulatory underbrush.”

Votes under the Congressional Review Act have come at such a rapid clip that liberal interest groups feel pummeled. After the House voted last week to overturn a planning rule issued by the Bureau of Land Management, Defenders of Wildlife spokeswoman Haley McKey issued a statement headlined: “The Congressional Review Act Claims Latest Victim.”

Meanwhile, the Trump White House is employing an executive tactic that dates to the Reagan administration: issuing a 60-day freeze on new regulations.

The tactic is a mainstay of new administrations. George W. Bush initially delayed 90 Clinton-era rules, and Barack Obama delayed, altered or rescinded more than two dozen of Bush’s actions.

Within a week of Trump’s inauguration, the new administration withdrew 24 significant rules that were about to be sent to the Federal Register for publication, regulatory analyst Curtis Copeland said. The new administration also delayed the effective dates of roughly 250 others, including 30 Environmental Protection Agency rules that were frozen in a single day, with no opportunity for public comment.

And although White House Chief of Staff Reince Preibus’s Jan. 20 memo called for a 60-day regulatory freeze, some regulations already are being delayed longer than that. An Agriculture Department rule tightening animal welfare requirements for organic livestock and poultry was just delayed from March 20 until May 19.

As a result, groups that had finally settled long-fought battles are feuding once again. Take the issue of the rusty patched bumble bee, whose population has shrunk 87 percent since the mid-1990s. On Jan. 11, the Obama administration declared it would be added to the endangered species list. Last Thursday — a day before those protections were set to take effect — the Trump administration said it would postpone the listing until at least March 21.

Ryan Yates, director of congressional relations for the American Farm Bureau, said the group is “pleased that the administration is taking a second look.” If the bee is declared endangered, he said, farmers in parts of Wisconsin, Illinois and Minnesota would be subject to severe penalties for killing or harming the insects through “normal farming operations” such as plowing and pesticide use. As an alternative, Yates said, the Farm Bureau is open to discussing a strategy for voluntary conservation.

But voluntary plans are inadequate, said Rebecca Riley, a senior attorney with the Natural Resources Defense Council. Riley said the group is weighing whether to challenge the delay, which was issued without the normal 30-day period for public comment.

“We don’t want to reach a point of no return for the rusty patched bumblebee,” Riley said in an email, “but further delay could dash our last, best chance to keep this bee around.”

Incoming agency officials are also signaling significant shifts in the way some industries are regulated. In November, the EPA sent out a lengthy request to nearly 20,000 oil and gas companies, asking them to gauge their emissions of methane within 60 or 180 days, depending on their facilities. Methane is a potent greenhouse gas linked to climate change.

Matthew Hite, who represents gas processors as vice president for government affairs at the GPA Midstream Association, called the request “unnecessary and duplicative” and estimated that complying would cost each processor nearly $3 million.

Since Trump took office, EPA officials have been granting companies that ask for it a 90-day extension. Several oil and gas officials said they expect the methane survey to be scaled back significantly or abandoned altogether.

Meanwhile, Michael S. Piwo­war, the acting chairman of the Securities and Exchange Commission, said he has instructed staff to determine whether it is “still appropriate” to require manufacturers to certify that they do not use minerals from conflict-ridden areas such as Congo, where armed groups accused of massive human rights violations profit from their trade.

Some major U.S. firms, including Intel and Tiffany & Co., have embraced the policy, but others have said complying with the disclosure rule is costly and complicated.

Lawrence Heim, managing director at Elm Sustainability, an auditing firm that consults on conflict minerals, said he has seen “a notable slowdown” in the demand for doing “due diligence” on the origin of minerals, as manufacturers apparently place bets that the rule will soon disappear.

Implementation of Trump’s Jan. 30 executive order will be left in large part to the White House budget director. Nominee Mick Mulvaney, a Republican congressman from South Carolina, could be confirmed as soon as this week.

A coalition of liberal groups that include Public Citizen, NRDC and the Communications Workers of America has challenged the order in court, calling it “arbitrary, capricious, an abuse of discretion, and not in accordance with the law.” They predicted the order would force the government to eliminate critical public protections.

Spicer called those claims “wildly inaccurate,” saying they make “a ton of assumptions . . . on what may or may not happen in the future.”

Whatever happens in court, Rep. Jared Huffman (D-Calif.) predicts Trump’s executive order will cause “complete chaos.” Huffman noted that the National Oceanic and Atmospheric Administration briefly declined to issue critical regulations for the opening of fishing seasons off both coasts, unnerving commercial fishermen who rely on the government to set the annual guidelines.

“Apparently members of the new administration don’t understand some regulations are critically important for the economic sector, and businesses depend on them,” Huffman said.

Some industries are openly worried about what the directive will mean for them. Commercial drone manufacturers, for example, waited four years for the Federal Aviation Administration to issue its first rule integrating drones into public airspace. The FAA has been planning to tackle bigger questions, such as whether drones may fly over people’s heads or travel long distances.

Brian Wynne, president of the Association for Unmanned Vehicle Systems International, said he fears that the answers to those questions will be delayed. The “current inaction,” he said in a statement, could prevent drone “operations, such as news reporting, disaster relief and public safety, from becoming a reality.”

Many companies, however, foresee huge benefits from the regulatory rollback.

Eric Myers, chief executive at Oil City Iron Works in Oil City, Tex., said he’s seen a flood of new orders since Trump took office. The company makes metal castings for equipment used in energy, mining, farming and transportation — industries expected to benefit from Trump administration actions.

“It’s not happening in a tidal wave,” Myers said in an interview, “but it is coming.”